1. Field of the Invention
The present invention relates to a driving apparatus for driving a speaker.
2. Description of the Related Art
Conventionally, various techniques for generating a bass tone from a small speaker have been proposed. FIG. 13 shows an example of a speaker driving circuit in which the technique of the Helmholtz resonance is combined with that of negative driving. According to the circuit, even when a small speaker is used, a bass tone can be efficiently generated. Referring to the figure, an inverting amplifier circuit 300 has a differential amplifier 20 of gain A. A speaker 1 and a resistor Rc are connected in series between the output end of the differential amplifier 20 and the ground. The junction between the speaker 1 and the resistor Rc is connected to an adder 26 through a feedback amplifier 25. The transmission gain of the feedback amplifier 25 is set to β which is a fixed value. The input signal Si is processed by a characteristic compensating equalizer 27, added to the output of the feedback amplifier 25 in the adder 26, and then supplied to the differential amplifier 20. An acoustic apparatus like this is disclosed in, for example, JP-A-1-302997.
In the circuit shown in FIG. 13, the output impedance Zo from the inverting amplifier circuit 300 is indicated by the following formula.Zo=Rc·(1−Aβ)  (1)
Therefore, it is seen that the output impedance Zo is a negative resistance.
When the electric resistance of the voice coil of the speaker 1 is defined as RL, the electric resistance RL functions as the damping resistance common to a speaker unit 4 and a motional impedance of a cabinet 6 using the Helmholtz resonance. Values of the each resistances are set so that the output impedance Zo in the formula (1) has a value which cancels the electric resistance RL of the voice coil of the speaker 1, i.e., −RL. Therefore, the speaker unit 4 and the cabinet 6 are separately constant-voltage driven so that a bass tone can be efficiently generated.
In the acoustic apparatus of JP-A-1-302997, however, the characteristics of the speaker 1 are largely varied or dispersed, and there is a possibility that oscillation may occur. Moreover, a non-linear operation is so hard that the control signal which is output of the feedback amplifier 25 can be distorted. When a complex control is to be performed, furthermore, the circuit is complicated, and the production cost is increasing.